(1) Field of the Invention
The present invention relates to an optical receiver, and more particularly to an optical receiver which is configured to receive an optical signal.
(2) Description of the Related Art
With recent variety of data flowing through a communication network and recent increase of users of the internet, the amount of data flowing through the network is growing more and more. To cope with the growth, it has been requested to expand the capacity and the scale of the network. In the field of long-distance communications, therefore, for the purpose of expanding the capacity of the network through the use of the existing optical fiber facility, the main focus is placed on the WDM (Wavelength Division Multiplex) transmission system.
This WDM transmission system is a system which is configured to multiplex lots of signals with their respective wavelengths into one fiber when those signals are transmitted. Hence, when the wavelength-division-multiplexed signal is transmitted, the amount of data flowing through the fiber is increased in proportion to the number of wavelengths to be multiplexed. Since the WDM transmission treats a large amount of data to be transmitted through one fiber, if some accident such as a disaster or a malicious deed is brought about in the fiber facility, the communication network is far more greatly impaired. For protecting the WDM transmission line, therefore, the WDM transmission line ordinarily includes a redundant configuration, that is, a working system line and a protection system line.
Traditionally, a system for automatically controlling dispersion compensation required in switching a line has been proposed on the assumption that the WDM transmission line has the redundant configuration. For example, refer to the Official Gazette of Japanese Unexamined Patent Publication 2000-236298 (paragraph numbers [0014] to [0017], FIG. 1).
However, in building the redundant configuration of the conventional WDM system, the redundant provision of switches for switching a transmission line in the transmitting station and the receiving station of the WDM transmission system, that is, the redundantly switching of a transmission line for each signal wavelength makes the system too large in scale because the system needs the same number of switches and transponders as the number of signal wavelengths to be treated. The massive system has a shortcoming that it is not reasonable in a variety of factors such as initial cost, running cost, maintenance, and reliability.
As another shortcoming, for realizing the redundant configuration as keeping the system less large in scale, it has been considered to provide a transmission switching function after the signal is wavelength-division-multiplexed and a receive switching function before the signal is wavelength-division-multiplexed. This provision of the switching functions also has a shortcoming that the receive amplifier needs a considerable length of time until the amplifier enters into a stable state after switched and thereby the main signal stays in an erroneous state for a relatively long time, because the amplifier greatly varies its dynamic range and makes its transient response too large.
On the other hand, the foregoing Japanese Unexamined Patent Publication No. 2000-236298 is dedicated for controlling the dispersion compensation in switching a signal wavelength. No consideration is given to the delay of the transient response of the receive amplifier occurring when the transmission line is switched and the number of components of the overall redundant configuration is reduced.